EVOH resin, which contains hydroxyl groups, forms crystal structure due to strong hydrogen bond. The crystal structure can inhibit oxygen coming into EVOH resin and thereby imparting an excellent gas barrier performance such as oxygen gas-barrier performance to EVOH resin. For this property of the EVOH resin, a multilayer film comprising EVOH resin layer as a gas barrier layer is utilized as a wrapping film for food and so on. There is known that the gas barrier property of the multilayer film comprising EVOH resin layer is impaired by subjecting a wrapped article with the multilayer film to retort treatment or the like treatment which exposes to hot water for long time. The reason for the depression of the gas barrier performance of the multilayer film is supposed as follows: water comes into the EVOH resin layer from the outer edge of the multilayer film during the hot water treatment to destroy hydrogen bonds in EVOH resin, thereby allowing oxygen to come into the EVOH resin layer.
As a method of suppressing the depression of the gas barrier performance by the hot water treatment, adding a hydrate-forming metal salt as a drying agent to EVOH resin is known. A technique is also known that polyamide-based resin is added to EVOH resin to improve a resistance to hot water treatment. For instance, the patent document 1 discloses in Examples 7 and 8 that a resin composition in which a partially or completely dehydrated carboxylic acid salt hydrate is added to a mixture of EVOH resin and polyamide-based resin. These Examples show that by adding dehydrated disodium succinate hydrate excellent gas barrier performance were secured after the hot water treatment and value of melt viscosity at 230° C. were kept almost constant for hours.
The patent document 2 suggests an EVOH resin composition containing partially or completely dehydrated polyvalent metal sulfate hydrate as a drying agent in addition to the mixture of EVOH resin and polyamide-based resin. The EVOH resin composition can secure gas barrier performance even after hot water treatment and can exhibit an improved melt-kneading property. In particular, the patent document 2 discloses in Table 3 and Table 4 that a resin composition containing partially or completely dehydrated magnesium sulfate hydrate showed a behavior decreasing viscosity compared to initial viscosity in melt viscosity behavior at 230° C. with time, and that the resin composition exhibited an excellent gas barrier performance even after hot water treatment, respectively.